Recently, a wireless transmission system and a wireless access system which employ the ROF (Radio over Fiber) technology to accommodate radio base stations for mobile communication, typified by cellular phones, in a control station using optical fibers is drawing attention. According to the ROF technology, radio signals are optically transmitted through optical fibers to permit modems, controllers, etc. to be collectively accommodated in a control station, thereby simplifying the configuration of the radio base stations to make the radio base stations compact. This can allow multiple radio base stations to be disposed along roads and in underground mall, tunnels and so forth.
FIG. 1 shows an example of the configuration of a wireless transmission system 8 employing the ROF technology. This wireless transmission system 8 has a communication terminal 181, a plurality of base stations 182a, 182b for relaying communication data for transmission and reception of radio signals to and from the communication terminal 181, and a host control unit 183 which controls the general communications over an optical fiber communication network 184 including the connected base stations 182.
The communication terminal 181 is configured to be able to be mounted on a vehicle or portable so that radio signals are to be exchanged with the base stations 182 provided in individual zones. That is, the communication terminal 181 includes a device which is installed in, for example, a personal computer or the like to execute data communication, but is generally a cellular phone or the like which enables voice communication. Particularly, the communication terminal 181 is configured as a small-sized and lighter device designed for portability.
As shown in FIG. 2, implemented in each base station 182 are an antenna 101 for transmission and reception of radio signals to and from the communication terminal 181, an antenna sharing unit 102 connected to the antenna 101, an electro-optic conversion section 103 and a photoelectric conversion section 104 both connected to the antenna sharing unit 102, and a WDM multiplexer (demultiplexer) 105.
When receiving a radio signal from the communication terminal 181 via the antenna 101, the base station 182a transmits the radio signal to the electro-optic conversion section 103 via the connected antenna sharing unit 102 where the radio signal is converted to an optical signal. Next, the generated optical signal is WDM multiplexed with the received optical signal and the resultant optical signal is optically coupled into a single optical fiber to be transmitted to the optical fiber communication network 184. At this time, the base station 182 performs analog modulation on an optical carrier which remains as a burst wave having the same waveform as the radio signal as shown in FIG. 1, regardless of the radio format, and transmits the optical signal to the optical fiber communication network 184. With regard to the optical analog modulation, generally, analog modulation or intensity modulation is often executed while optical SSB modulation, optical FM modulation or optical coherent modulation (PM modulation) may be carried out to overcome various problems.
The optical signal transmitted to the optical fiber communication network 184 from the base station 182a is subjected to modulation/demodulation process under the control of the host control unit 183, and is then transmitted to another backbone network 190 or another base station 182b. In the base station 182b, the optical signal received over the optical fiber communication network 184 is demultiplexed by the WDM multiplexer (demultiplexer) 105, the demultiplexed signal is converted to an electrical signal by the photoelectric conversion section 104, and the electrical signal is then transmitted as a radio signal to the communication terminal 181 via the antenna sharing unit 102 and the antenna 101.
In other words, the wireless transmission system using the ROF technology does not need to execute modulation/demodulation of a radio signal at a base station. This makes it unnecessary to change the system configuration of a base station even if the services and the specifications are changed, so that the flexibility of the whole system can be improved.
The ROF is used as means for constructing a low-cost remote antenna which is provided as a countermeasure for dead zones where radio waves, such as broadcast waves, do not reach, and is particularly suitable when constructing a low-cost system under a picocell environment. Particularly, the ROF can permit a variety of radio formats to be handled by the same base station device in a unitary manner and it is attracting attention as a technology that can flexibly and quickly cope with a future change in radio format.
Conventionally, a technique disclosed in, for example, Japanese Unexamined Patent Publication No. 2005-175675 is proposed as a radio transmission station using the ROF technology. According to the disclosed technique, the sender's portable communication device expands the bandwidth of radio transmission signals according to the bandpass characteristic which is opposite to the bandwidth limiting characteristic of a bandwidth limiting filter provided in a receiver's portable communication device, and transmits the bandwidth-expanded radio transmission signals to an optical transmission path. This can effectively attenuate the spurious that occurs in an optical transmission zone without degrading the characteristic of a desired signal.
The radio transmission station using the ROF technology is also adapted to, for example, a communication system between a road and a vehicle (see, for example, Japanese Unexamined Patent Publication No. 2002-33694). The road-vehicle communication system constructs a network with base stations disposed along a road, antenna sections of the base stations, a control section which controls the base stations, and cables which connect those components. Radio transmission between a mobile terminal, mounted on a vehicle, and a base station is carried out based on the code division multiplex transmission system, and the control section collectively controls individual processes, such as communication modulation and demodulation processing, multiple access processing, frequency assignment and code assignment. This can minimize complex control accompanying the movement of a mobile body like a vehicle.
In the wireless transmission system 8 using the ROF technology, it is ideal that the optical fiber communication network 184 can be constructed by an IP (Internet Protocol)-based existing optical network, not an exclusive analog optical link transmission path. The IP network is a connectionless network which executes packet transfer determined by the IP. In the IP network, an address indicative of the transmission destination is inserted in the header of each packet as divided transmission data, and a router 186 transfers the packet to the transmission destination based on the address.
At this time, the router 186 identifies the IP address and MAC (Media Access Control) address located in the header of the packet transmitted over the IP network, and a tag or label also added to the header, and sends the packet to a transmission path to the destination according to a routing table implemented in the router 186. As an example of the router, a low-cost L2/L3 switch has been frequently used in place of the conventional legacy router. The IP network is constructed by arranging access nodes in a mesh pattern using the L2/L3 switches. It is expected that a packet-exchange type mesh network constructed by similar L2/L3 switches will be the mainstream in a wider trunk network.
Even in a somewhat wide trunk network, a low-cost mesh network can be constructed by replacing existing routers with L2/L3 switches if data formats to be handled can be limited to an IP packet and a frame based on the standardized Ethernet (registered trademark) protocol.
By way of contrast, in recent years, particularly, a so-called VPN (Virtual Private Network) is becoming popular, in which end-to-end communication or subnet-to-subnet communication is virtually considered as one LAN (Local Area Network) separated from other communication networks. In the VPN, a private network is constructed on a shared network, and data to be exchanged with a communication party is encrypted to prevent a third party from seeing the communication contents.
To efficiently exchange packets in the IP network and transmit them to a communication party in the VPN, the MPLS (Multi-Protocol Label Switching) technique, tag VLAN (Virtual LAN), etc. are proposed as a label switching technique, for example. This tag VLAN constructs a virtual network group by combining arbitrary terminals without depending on the apparent connection mode in networks including wireless LAN/cable LAN.
The wireless transmission system using the ROF technology generally uses an analog modulation system for optical modulation. When intensity modulation is used in the optical modulation, particularly, the modulation is likely to be affected by the non-linearity of optical devices used in transmission systems, such as the electro-optic conversion section 103 and photoelectric conversion section 104 in FIG. 2. When information is transmitted by the ROF, therefore, optical devices and optical transmission devices which are used in the ordinary fast optical digital communication cannot be adopted directly, and high-performance optical devices which do not have non-linearity, and an advanced non-linear correcting technique are needed for the ROF, which leads to a significant increase in the cost of the overall system.
In particular, the recent popularity and progress of FTTH (Fiber To The Home) allow optical fiber networks to be constructed at ends of offices as well as at individual homes. However, such an optical fiber network is an optical fiber access circuit designed for cost reduction and optimization as an IP network to provide a broadband circuit, and makes it difficult to use transmitters/receivers, exchangers, etc. to be used therein as a wireless transmission system using the ROF technology.
In the conventional ROF, the signal format in an optical fiber was an analog modulation system, and suffered poor compatibility with a digital optical transmission device which became popular according to the global use of the FTTH and optical Ethernet (registered trademark) network even if real data to be transmitted was IP packet data. In the case of executing the ROF-based communication, therefore, conventionally, the control station should always execute demodulation to enable packet exchange.
Conventionally, when one wants to construct a virtual private network suitable for wireless access, i.e., when signals are exchanged between wireless communication terminals belonging to radio access zones which are controlled by different base stations or the control station, signals received from one communication terminal needs to be demodulated once by the base station or the control station or the like. An IP packet demodulated by the base station or the like transmitted to the desired base station or the control station over the IP network and is then modulated to be transformed into a radio signal, which is in turn transmitted to a radio zone via an antenna. Accordingly, the modulation and demodulation processes that are executed by a base station and the control station become redundant, and wasteful traffic in the IP network increases, eventually increasing the total amount of traffic.
Further, while a one-segment broadcasting service of the terrestrial digital broadcasting targeted for reception by mobile terminals is becoming popular, radio wave dead zones exist not only within a building, a train station and the like, but also some locations outdoor. To eliminate such radio wave dead zones, the use of a system of re-relaying broadcast waves via a gap-filler system indoor and outdoor is in process. While studies are made to use the ROF as the gap-filler system, such approach requires new devices and organization of the infrastructure, which would lead to consumption of a substantial time and cost.